This invention pertains to active matrix displays comprising organic light emitting elements.
Displays that comprise smart pixels are known. Typically, a smart pixel comprises a light-emissive element and a circuit that comprises one or more field effect transistors (FETs) which drives/switches the emissive element. A given pixel typically is addressed by several conductor lines which typically are connected to peripherally disposed drive circuitry.
Recently organic light emissive elements (typically organic light emitting diodes; see, for instance, A. Dodabalapur, Solid State Communication, Vol. 102, No. 2-3, pp. 259-267, 1997) have been disclosed, and have been proposed for use in displays. See, for instance, M. K. Hatalis et al., Proceedings of the SPIE, 3057, p. 277 (1997), and C. C. Wu et al., IEEE Electron Device Letters, Vol. 18, p. 609 (1997). The references disclose smart pixels with organic light emitting diodes (LEDs) and field effect transistors (FETs) with, respectively, polycrystalline and amorphous Si active channel material.
Furthermore, displays that comprise organic smart pixels have also been proposed. See, for instance, Dodabalapur et al., Applied Physics Letters, Vol. 73(2), July 1998, pp. 142-144, and U. S. patent application Ser. No. 09/087,201, filed May 29, 1998 by Bao et al. See also H. Sirringhaus et al., Science, Vol. 280, page 1741, Jun. 12, 1998. In such a display, a given pixel not only comprises an organic light emitting diode (LED) but also one or more organic pixel FETs.
Active matrix displays with organic LEDs and organic pixel transistors potentially have significant advantages, e.g., low cost and compatibility with flexible plastic substrates.
We have come to realize that components such as organic LEDs and organic pixel FETs frequently exhibit certain limitations and/or non-ideal characteristics (collectively xe2x80x9cnon-idealitiesxe2x80x9d) that can adversely affect the performance of otherwise potentially excellent displays.
For instance, we have discovered that charge carrier mobility and/or threshold voltage of organic LEDs frequently change slowly with time, that charge carrier mobility and/or threshold voltage of organic FETs frequently vary from FET to FET, and that organic pixel FETs frequently are subject to capacitive signal feedthrough through the gate insulator and to charge leakage because of standby currents when transistors are off. These and other non-idealities can result in displays with significant brightness variations and/or other shortcomings. Such variations will frequently be unacceptable, especially in view of the known sensitivity of the human eye to brightness variations. FETs with polycrystalline or amorphous Si active channel material also frequently exhibit non-idealities.
In view of the potential advantages of active matrix displays with organic smart pixels, it would be highly desirable if at least some of the non-idealities could be mitigated or eliminated. This application discloses some significant non-idealities, and also discloses means for overcoming them.
The following U. S. patents and applications pertain to related subject matter: U.S. Pat. Nos. 5,405,710; 5,478,658; 5,574,291; 5,625,199; and 5,596,208; application Ser. No. 08/441,142, filed May 15, 1995 by Dodabalapur et al; application Ser. No. 09/087,201, filed May 29, 1998 by Bao et al; and application Ser. No. 09/137,920, filed Aug. 20, 1998 by Dodabalapur.
All references that are cited herein are incorporated herein by reference.
In a broad aspect the instant invention is embodied in an active matrix display wherein a given pixel comprises at least one organic component, typically an organic LED. The pixel typically further comprises at least one organic or Si-based pixel FET (e.g., polycrystalline Si FET or amorphous Si FET). Associated with the presence in the pixel of one or more organic, polycrystalline Si or amorphous Si components are some non-idealities.
There are at least two types of non-idealities. One type is due to non-ideal device characteristics of the organic transistors and requires corrective action for each smart pixel, typically at the frame frequency (exemplarily about 75 Hz). Exemplary of the first type of non-ideality are capacitive signal feed-through through the gate insulators of organic pixel FETs by short rise/fall time pulses and charge leakage due to relatively low on-off ratios of organic transistors.
The other type of non-ideality is due to, typically slow, changes in physical characteristics (e.g., mobility, threshold voltage) of the organic components, and requires only intermittent corrective action (e.g., when the display is activated, and/or at predetermined intervals that are much longer than the frame period, for instance, once a day).
In order to mitigate or overcome some or all of the non-idealities, a display according to the invention comprises circuitry, at least part of which is typically disposed in the periphery of the display, that inter alia performs various compensatory functions. This circuitry will be referred to as the xe2x80x9cdrive/compensationxe2x80x9d circuitry.
Drive/compensation circuitry for mitigating the first type of non-idealities will typically comprise additional FETs (i.e., FETs in addition to the conventional pixel FET) that act to mitigate or eliminate, for instance, the capacitive signal feed-through, charge leakage or other non-ideality of prior art smart pixels. The drive/compensation circuitry for mitigating the second type of non-ideality will typically comprise means for periodically measuring and storing appropriate characteristics of each smart pixel (exemplarily the voltage that is required to produce a certain current through the LED, and/or the threshold voltage). This information typically is stored in an electronic memory, and the drive/compensation circuitry adjusts the drive conditions of each pixel that deviates from target conditions, taking into account the traits of the individual pixels.
Those skilled in the art will recognize that the above-described approaches to mitigation of smart pixel non-idealities are, inter alia, possible because the precision and accuracy of conventional Si-based circuits typically are much greater than those of organic-based circuits. Thus, at least part of the drive/compensation circuitry according to our inventive is preferably embodied in Si technology, typically conventional C-MOS technology.
Among the non-idealities of pixels with one or more organic components typically are
a) variations in mobility and/or threshold voltage of the organic pixel FETs from transistor to transistor;
b) change in mobility and/or threshold voltage with time in a given pixel FET;
c) change over time of the LED characteristics;
d) capacitive signal feed-through through the gate insulator of the organic pixel FETs by short rise/fall time pulses; and
e) charge leakage through the gate dielectric due to poor on-off ratio of the organic pixel FET.
Of the above-cited non-idealities, non-idealities a), b) and c) typically require corrective action at a frequency much below the frame frequency of the display, and non-idealities d) and e) typically require corrective action for each pixel at the frame frequency. The former will frequently be referred to as xe2x80x9cadaptive pixel controlxe2x80x9d.
More specifically, the invention exemplarily is embodied in display apparatus that comprises a multiplicity of nominally identical smart pixels disposed on a first substrate region, and that further comprises a smart pixel-free second substrate region. A given smart pixel comprises an organic light emitting diode, and pixel circuitry for providing a current through the organic light emitting diode. The pixel circuitry of the given smart pixel comprises at least one pixel FET (typically, but not necessarily, an organic pixel FET) in series with the organic light emitting diode and disposed in the first substrate region.
Significantly, the nominally identical smart pixels unintentionally exhibit one or more non-idealities that adversely affect the performance of the display apparatus. The display apparatus further comprises drive/compensation circuitry selected to at least mitigate said one or more non-idealities, such that the performance of said display apparatus is improved.
Typically, the field effect transistor in series with the organic LED is an organic FET (but could be a polycrystalline or amorphous Si FET), and the drive/compensation circuitry typically comprises single crystal Si (exemplarily conventional C-MOS) circuitry.
By way of example, the drive/compensation circuitry is selected such that compensating charge injection into the gate terminal of the organic FET mitigates capacitive signal feed-through or such that setting an inactive high value of a ROW signal and a RST signal to a value above a supply voltage Vdd mitigates charge leakage.
By way of further example, the drive/compensation circuitry is selected to measure and store one or more characteristics of each smart pixel, and to make, if indicated by the result of the measurements, a change in the control voltage such that substantially all smart pixels have substantially the same light emission for a given signal provided to the display apparatus.